The present invention relates to a method of forming a metal image in a desired pattern on a substrate. Two different photoimageable film layers are provided on the substrate, such that after exposure to actinic radiation to produce a negative of the desired image and subsequent development, a resulting “umbrella” is formed of the top photoimageable layer over the bottom photoimageable layer. A metal layer is subsequently deposited over the photoimageable film pattern on the substrate. The “umbrella” allows for the clean removal of the photoimageable film layers, without damage to the subsequently applied metal.
The invention is usable in processes such as the manufacture of printed wiring boards. A typical process for the manufacture of a printed wiring board is described in U.S. Pat. No. 6,044,550 to Larson et al., the subject matter of which is herein incorporated by reference in its entirety. The invention is also usable in a variety of other processes that have similar steps to the process described below for formulating a metal image on the substrate.
A typical procedure for making a metal image, on a substrate is as follows:    1) A dry film or other photoimageable layer is laminated or otherwise applied to the substrate.    2) A negative image is produced using actinic radiation on the photoimageable layer covering the substrate.    3) The unexposed photoimageable layer is developed off using a mild alkaline solution, such as 1% K2CO3, leaving the cleared substrate and the exposed resist.    4) The entire substrate is coated with a conductive metal. Preferably, a layer of gold is sputter coated onto the substrate.    5) As a result of step 4, conductive metal (gold) covers both the substrate and the photoimageable layer.    6) The remaining photoimageable layer is then stripped from the substrate using an alkaline solution, such as a 3% solution of KOH.    7) Finally, with the remaining photoimageable layer removed, the gold image remains on the substrate.
Suitable processes are described, for example, in U.S. Pat. No. 5,733,466 to Benebo et al., the subject matter of which is herein incorporated by reference in its entirety.
A major concern with the above described procedure is that the gold is coated up and over the photoimageable layer (dry film) pattern on the substrate (i.e., traces) in one continuous film. When the dry film is stripped off, the gold is ripped apart at the edges where the resist and the gold meet on the substrate surface. The result is a potentially ragged line of the gold remaining after stripping, and potential lifting of the unwanted resist pattern from the substrate.
In the case of the manufacture of printed wiring boards, when lifting occurs, there is potential to create unwanted plating between features on the printed wiring board, causing them to be electrically shorted or near shorted. Thus, it would be highly desirable to develop a manufacturing method that results in straight traces on the substrate, without ragged tear lines.
One way that has been suggested to avoid the ragged metal is to develop the dry film so that it has a severe negative foot, or under cut, near the surface of the substrate, thereby resulting in an “umbrella-effect” whereby the sputtered gold, deposited from above, would not coat directly up to the edge of the dry film/substrate interface. The disadvantage of such a severe negative foot is that it is difficult to consistently reproduce and can lead to resist failure, or lifting of the resist. This lifting can cause a short of the sputtered gold. Additionally, a negative foot may be non-uniform along the substrate-dry film interface.
Thus, there remains a need in the art for an improved method of producing gold traces on the surface of the substrate, that does not result in a ragged line of the gold trace remaining after the photoresist is stripped from the substrate.
The inventors have surprisingly discovered that the use of two layers of photoresist each having different break times and/or developing times and/or curing speeds produces a sandwich of the two layers, that has an “umbrella” effect of the top layer of photoresist over the bottom layer of photoresist. This “umbrella” or T shape of the photoresist sandwich significantly eliminates ragged lines along interface between the substrate and the dry film photoresist, which were common in the prior art.